My personal preference is to place the majority of my implants behind the muscle. I do that for a number of reasons. I feel that it gives the implant a more natural look and feel, you get a more tissue coverage. The more of the patients body you can have between the skin and the implant, the more natural those implants are going to look and feel.
The muscle also provides some level of support for the implant, so it will give a longer lasting result in most cases. Rare instances where I'll place implants over the muscle is patients that are performance athletes, patients that are professional body builders or are flexing where you have the risk of seeing movement or deformation of the muscle when they're working.